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Discussion Starter · #1 · (Edited)
Given that most target curves are at least +6db in the affected region, looking at this graph I'd think that an underdamped Q of 2.0 would be beneficial, but everything I read seems to be in opposition to that, describing it as "high efficiency, with bad transients and bad frequency response".
Conversely, the 'ideal' Q of .707 seems like it'd require a lot more EQ to match the target slope.
I can see how it'd be preferable in a home audio environment when shooting for a flat response, but not in a car.
What don't I understand?

Slope Rectangle Font Parallel Pattern
 

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Only educated thing I can chime with here is that this graph takes no cabin gain into account - which has dramatic effect on the response of your subs, regardless of the qtc of your enclosure.

I am positive @daloudin would be far more qualified to help here with more detail.
 

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You also get really ugly phase shift in the area around the peak. And while eq could fix the peak, you will require a lot more power and excursion to create the same output at lower frequencies with an Fb that high.

Linkwitz explored this idea in great detail and actually developed a method to get a smooth curve in a tiny box, but again output is limited and power requirements increase exponentially.

 

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Also, that curve is drawn incorrectly which could easily lead to incorrect conclusions. The .707 and .5 curves should not start sloping down earlier than the high Q alignments. The peaks for higher Qtc alignments should all be shifted right proportional to the amount of increased Q.

For a notional driver with QTS of 0.4, Vas of 100L and fs of 27, here’s where you’d start seeing the drop off start (fb = -3 dB):

qtc. F3
0.5 60 hz
0.7 71 hz
1.0 90 hz
1.5 127 hz
2.0 165 hz
 

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Regardless of the frequency bump, it also doesn't sound as good. That being said using the xover or even a high pass filter can help ameliorate some of the bumpThat means you can cheat a little with filters and using a small box, jsut don't get carried away as a Q that's way off in one direction will also sound bad, I would expect to hear it ringing a little bit or sounding muffled and weak. Another trick to small boxes is a aperiodic port.
 

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Given that most target curves are at least +6db in the affected region, looking at this graph I'd think that an underdamped Q of 2.0 would be beneficial, but everything I read seems to be in opposition to that, describing it as "high efficiency, with bad transients and bad frequency response".
Conversely, the 'ideal' Q of .707 seems like it'd require a lot more EQ to match the target slope.
I can see how it'd be preferable in a home audio environment when shooting for a flat response, but not in a car.
What don't I understand?

View attachment 348877
Depending on room size and cone area the higher Q alignments are sometimes better for Home Theater if you can get that peak low enough to supplement the natural room response. We're talking multiple 18+" drivers with Fs and F3 response in the Infrasonic range. In fact, a number of THX and IMAX theaters that have limited square footage do this to help with untrained personnel who may push the volume beyond what a more natural or lower Q alignment could handle without mechanical damage.

For car audio, my personal preference is sealed around 0.6 so that the combination of roll off and cabin gain makes for a more "Natural" response curve. By the time you add +12dB/Octave cabin gain and +6dB of box gain from a Hi-Q box, you have a peak that can NOT be EQ'd out - now if that peak is where YOU want it to be and you understand that from the get-go, then there's nothing wrong with that... or if you want maximum transients versus extension then using a 0.9 (or more) enclosure and letting the cabin gain fill in the more steeply rolled off bottom end then that's an option as well. But for the most natural response and SQ then the sealed to pseudo IB setup that fully leverages your particular setup and vehicle cabin gain is the simplest way to get the "flat" response preferred by most SQ enthusiasts (especially if you're looking for palpable output in the low 20's) with the ability to have enough output to compensate for tire rumble, road and wind noise at speed.

So to answer your question, High Qtc is not in and of itself, "bad" per say, if you understand the setup and are getting the results you want. It's also one of the few things that can be modeled relatively easily with freeware and why it is the most often discussed. Midbass is the hardest to predict in car audio due to the dimensions and restricted driver placement. Then the Midrange to Highs are relatively easy due to the limited distance from the drivers and the closed environment (up to the point of diminishing returns and the reflections and acoustics of your particular install.)
 
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Discussion Starter · #8 ·
Depending on room size and cone area the higher Q alignments are sometimes better for Home Theater if you can get that peak low enough to supplement the natural room response. We're talking multiple 18+" drivers with Fs and F3 response in the Infrasonic range. In fact, a number of THX and IMAX theaters that have limited square footage do this to help with untrained personnel who may push the volume beyond what a more natural or lower Q alignment could handle without mechanical damage.

For car audio, my personal preference is sealed around 0.6 so that the combination of roll off and cabin gain makes for a more "Natural" response curve. By the time you add +12dB/Octave cabin gain and +6dB of box gain from a Hi-Q box, you have a peak that can NOT be EQ'd out - now if that peak is where YOU want it to be and you understand that from the get-go, then there's nothing wrong with that... or if you want maximum transients versus extension then using a 0.9 (or more) enclosure and letting the cabin gain fill in the more steeply rolled off bottom end then that's an option as well. But for the most natural response and SQ then the sealed to pseudo IB setup that fully leverages your particular setup and vehicle cabin gain is the simplest way to get the "flat" response preferred by most SQ enthusiasts (especially if you're looking for palpable output in the low 20's) with the ability to have enough output to compensate for tire rumble, road and wind noise at speed.

So to answer your question, High Qtc is not in and of itself, "bad" per say, if you understand the setup and are getting the results you want. It's also one of the few things that can be modeled relatively easily with freeware and why it is the most often discussed. Midbass is the hardest to predict in car audio due to the dimensions and restricted driver placement. Then the Midrange to Highs are relatively easy due to the limited distance from the drivers and the closed environment (up to the point of diminishing returns and the reflections and acoustics of your particular install.)
Fantastic response. Thanks.
 

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Given that most target curves are at least +6db in the affected region, looking at this graph I'd think that an underdamped Q of 2.0 would be beneficial, but everything I read seems to be in opposition to that, describing it as "high efficiency, with bad transients and bad frequency response".
Conversely, the 'ideal' Q of .707 seems like it'd require a lot more EQ to match the target slope.
I can see how it'd be preferable in a home audio environment when shooting for a flat response, but not in a car.
What don't I understand?

View attachment 348877
A couple of things cause it to be less than ideal in most circumstances.
1) In a car you have cabin gain helping out with the lower end as others have explained.
2) You can end up with distortion caused by the non linearity of the air spring of the box. The speaker is going to have an easier time expanding the air within the box vs compressing it. So the forces on the front and back of the cone will be unequal. Typically this is compensated for with suspension and motor design.
3) The motor won't be able to control that non linearity, causing impulse ringing where the driver won't stop moving after the signal is stopped.
 
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